Large-area laser processing for aerospace

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Anibal Di Luch, of TWI, reports on the development of a robotic platform for the laser processing of large-scale, high-end structures for aircraft

Laser processing has the potential to enable higher productivity manufacturing of both metallic and composite aerospace structures.

This can be achieved in several areas. For example in fabrication, lasers can be used to weld structures, the best example being stringer-to-skin panels, while laser additive manufacturing can be used to produce large metallic structures such as engine components and wing ribs. For material preparation, laser surface engineering can be used to improve aerodynamics and/or minimise ice formation in leading-edge structures, such as wings, fan blades and lip skins. And lastly during assembly, lasers can be used to very accurately cut and drill panels, especially those with complex curvilinear geometries.

However, as a remotely operated process, the lack of large-scale, reliable and cost-effective manipulation of laser technology has limited its applicability when processing large aerospace structures.

A number of different types of beam/ workpiece manipulators are used for industrial laser welding. These can be classified into articulated-arm manipulators (i.e. robots), multi-axis table and gantry systems, and laser beam scanners. Robotic manipulation is the most 3D-flexible, but with lower accuracy and a working envelope limited by arm reach. Gantry and table systems are more accurate, but are more limited in 3D application, and are often more costly. Scanners are free of moving mechanical parts, meaning both very high speeds and accuracies can be achieved. Scan fields are normally 2D, and size limited, meaning only up to medium-sized components (for example car doors) can be welded, unless a robot-mounted scanner is used.

In the UK-Sweden (Vinnova/Innovate UK) funded ‘LaserTau project’, TWI and the other project partners have worked together to integrate a novel high-speed, accurate (<10μm) and modular robot with laser processing capability for the first time. This platform can demonstrate the ability to manufacture large-scale, highend aerospace structures – the prototype system has a completely scalable working envelope of around 3m x 1m x 1m. Parallelkinematic machines (PKMs), investigated in LaserTau, are an attractive middle ground, having the 3D flexibility of a robot with the accuracy of a gantry or table, see figure 1.

Figure 1: Manipulator performance comparison

The Tau robot platform combines and optimises the accuracy, precision and stiffness of a gantry system, the speed and mobility of a galvo-scan unit and the 3D flexibility of an articulated-arm manipulator. These features are further enhanced by the tunnel-shaped architecture of the platform. The above allows configuration for larger working envelopes in three dimensions, more readily than other manipulation systems can provide.

The LaserTau project is a collaboration between TWI and CAV Advanced Technologies (CAV) from the UK, and Prodtex, Cognibotics and Corebon from Sweden.

TWI is primarily leading the project as a technology enabler and system validator, having extensive experience in laser processing, and will drive market adoption of the developed platform through demonstration to an industrial audience.

CAV, as a Tier 1 aerospace supplier, has been bringing market knowledge, setting the overall LaserTau system specifications in line with the selected aerospace case study to prove the concept.

Prodtex is a development company, and is in charge of the overall design of the LaserTau system structure, virtual modelling, nominal kinematics and geometric analysis, and offline programming of the toolpath, including integration with the laser process head.

Cognibotics is contributing by providing a PKM-suitable robot motion controller. Due to earlier prototypes and demonstrations, this challenging task can be accomplished with great confidence, building on earlier experiences.

Corebon is specialised in the design and manufacture of bespoke lightweight carbon fibre reinforced plastic (CFRP) components, which are used in the Tau robot platform.

Incorporating Industry 4.0

Prodtex has designed and patented an innovative reconfigurable, modular steel framework called ‘BoxJoint’, which is used as the main frame structure for the Tau robot.

The system has the following benefits:

  • An adjustable robot working volume through adding or removing modular framework;
  • Re-usable modular components;
  • Quick design time and short lead assembling time;
  • Increased stiffness compared to aluminium alternatives; and
  • Easy to reconfigure, calibrate and transport compared to permanent solutions.

In kinematic structures, it is essential to bring down the resilient mass of the constituent components in order to obtain accuracy and performance at high processing speed and acceleration/deceleration.

Prodtex has used 3DExperience software to create the digital twin of LaserTau, which permits a quick visualisation of the laser welding platform in the processing cell before commissioning the actual physical system. The digital twin (see figure 2) has enabled, among other things, the optimisation of the processing cell layout, the maximising of the working envelope of the system, and the accurate simulation of laser manipulation operations, for example to identify any laser processing constraints. In this sense, Delmia Arc Simulation software has been used to determine operations and arc-trajectories, which are then translated and exported into the customer required G-code to then be used in the robot controller.

Figure 2: The LaserTau processing platform (left) and its digital twin (right)

Design, development and validation through a single collaborative platform has allowed for complete digital continuity and connectivity in line with Industry 4.0 standards. Using Delmia Equipment Design software and its automatic kinematic solver, the inverse kinematics were easily established and with the simple resource and port connection management, setting up of the laser processing cell is intuitive.

Corebon has designed lightweight, high-stiffness CFRP hardware, including ball joints, arms and an end effector fixturing dome (that is, structural parts affecting overall stiffness when subjected to dynamic loads). Corebon’s structural component range consists of standardised plates, tubes, profiles, and beams with an unprecedented strength-to-weight ratio. Corebon’s production process of these parts, which is highly automated, results in consistent high-quality components. The parts developed and manufactured by Corebon have in general superior strength/ stiffness to weight ratio compared to conventional possible solutions in similar applications. The parts have been, within the framework of this project, thoroughly tested in order to ensure the quality in terms of stiffness, wear and endurance, to satisfy high processing performance.

The bespoke Tau robot control system

Typically, robots are serial-kinematic machines, including those used in laser welding. Such robots, while versatile, have the drawback of error multiplication and do not scale-up to cover large workspaces without adding linear rails and thereby further motion error factors. Cognibotics has developed a PKMsuitable robot motion controller. The LaserTau robot provides six degrees of freedom with no motors close to the end-effector – the physical part of the robot where the laser processing head is attached. By means of the flexible BoxJoint framework, both the current horizontal configuration and an earlier vertical configuration have been built. For laser applications with operation in a horizontal configuration, workpiece and fixture placement is simplified since the items can be brought directly into the robot cell by normal transportation, followed by automated touch-up of the workpiece.

The developed robotic platform will enable lasers to be used in the manufacture of large-scale aerospace structures.

The resulting control system is running TwinCAT software, which enables multiple ways to interact with the robot platform and has the ability to integrate the robot with other components. The EtherCAT interface enables high performance IO and integrated laser process control that adhere to existing safety standards. Modularity and scalability (in length, strength, etc.) are inherent properties of the mechanics (framework, linear guides, PKM-links), the controller hardware (IO, servo drives) and the software (G-code programming, PLC functions and calibration support), paving the way for numerous applications in aerospace manufacturing and elsewhere.

Summary

The combination of industrial laser sources with the Tau robot provides a novel platform – as a result of the acceleration, accuracy and repeatability achievable – to further the state-of-the-art in laser welding, laser cutting, laser surface engineering, laser drilling and laser additive manufacturing. Within the LaserTau project, TWI has provided excellence expertise in laser processing, validating the Tau robot platform against CAV case study requirements.

Furthermore, the following outputs have been covered:

  • Supported the integration of the Tau robot and at least two relevant industrial laser sources, in order to ensure the relevant international standards are adhered to (e.g. IEC 60825); and
  • Working with CAV, provided inputs to address the aerospace sectors' future requirements from a materials processing perspective. This knowledge will ensure that the LaserTau system is relevant to a broad range of applications.

LaserTau provides unparalleled capability in combined accuracy, repeatability and acceleration. Combined with state-of-theart industrial laser sources, the LaserTau system will enable a cost-effective solution to manufacture laser welded components, and next generation laser applications (ultra-high-speed picosecond laser surface engineering, for example).

CAV is the end-user partner in LaserTau and will be the potential first adopter of the novel LaserTau technology.

While this product is currently being investigated for use in the aerospace industry, it is capable of processing large structures/components in other industry sectors. For instance, in the future, it could be dismantled, re-configured and reassembled over the period of a few working days to adapt to changes in product location, size and geometry and in this way, LaserTau can be deployed in-situ to repair large components.

Anibal Di Luch is a project leader in laser welding and sheet processing at TWI.

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